Energy distributions of particles ejected from laser-generated pulsed plasmas

A study of laser ablation in vacuum of metallic elements (Al, Cu and Ta) at different boiling points is reported. A Nd:YAG laser radiation, 3 ns pulse duration and 10⁹ W/cm² intensity, produces non-isotropic emission of neutral and ionic species. Mass quadrupole spectrometry, coupled to a 45° electrostatic ion deflection, allows estimation of the energy distributions of the emitted species within the plasma plume as a function of the incident laser energy. Neutrals show typical Boltzmann distributions indicating the plasma temperature. Ions show Coulomb-Boltzmann-shifted distributions, typical of plasma thermal interactions, adiabatic expansion in vacuum and Coulomb interaction between charged species. Surface profiles of the craters and microscopy investigations permitted to study the ablation threshold, the ablation yields and the deposition rate of thin films on silicon substrates. The multi-component structure of the plasma plume emission is described in terms of charge state, ions and neutrals temperature and plasma density. A special regard is given to the study of the ion acceleration process occurring inside the plasma due to a high electrical field generated in the non-equilibrium plasma.